It is a modern and advanced transportation mode to take a container as a unit for transportation. Containerization has become the main trend of the international cargo transportation. Meanwhile, the uses of containers in smuggling firearms, weapons, drugs, explosives, even WMDs (Weapons of Mass Destruction) and RDDs (Radiological Dispersal Devices) have become an international public hazard troubling every government and interfering with the normal order of the international cargo transportation.
Since 9.11 in U.S., the U.S. government began to attach great importance to the potential risks of the cargo transportation, and worried more about WMDs and RDDs through containers into the United States. To guard against such risks, the U.S. Customs Office issued a “Container Security Initiative (CSI)” in Jan. 17, 2001, in which all the foreign ports with transactions directly linked to the U.S. port were required to be equipped with non-invasive X(γ)-ray scanning imaging equipments for performing ray-scanning inspections on the containers shipped to the U.S. One year after CSI being announced, there were 18 major ports in the world which joined the initiative and began to operate. In the background where the requirements for the international transportation safety are increasing, the World Customs Organization unanimously adopted a resolution calling for all the 161 members to develop a plan regarding the container security inspection along the mode of the CSI, i.e the container security inspection has become a world's common topic of concern.
The existing container X(γ)-ray security inspection equipments mainly aim at transmission imaging, i.e. to directly penetrate cargo using X-rays so as to obtain transmission images of all the articles covered by the path of the X-rays. The standard transmission imaging technology solves the “visualization” problem of containers, such that it has been widely used. A dual-energy transmission technique is developed from the transmission imaging technology. In the dual-energy transmission technique, X-rays having two different energy spectra transmit the inspected article. The acquired differences between the signals of the X-rays are processes to obtain atomic number information of a material in an article being inspected. This technology can effectively improve effect of security inspection to a certain extent, and the requirements proposed by CSI for the trend from inspection for smuggling to inspection for dangerous articles can be met. A substance identification system adopting this technique has been successfully implemented in a high-energy X-ray dual-energy imaging container inspection system, with advantages of high operating speed, accurate identification and so on.
The substance identification subsystem in the high-energy-X-ray dual-energy imaging container inspection system (referred to as “high-energy dual-energy system” below for short), however, has the following disadvantages.
An individual mark procedure is necessary to obtain a set of classification parameters suitable for a system before usage because the dual-energy state of each system is different from another. The marking is one of the most complicated processes during testing a dual-energy system.
Furthermore, the dual-energy state will fluctuate in an accelerator subsystem in a high-energy dual-energy system for various reasons such as a long haul, exchange of elements, a dose adjustment by human operator, and so on. Re-mark is needed when the fluctuation is significant.
Moreover, the dual-energy state of the accelerator such as pulse current fluctuation and state fluctuation due to AFC will vary over time even for the same accelerator, which is caused by the limitation of the accelerator technique. Due to the varying of the state, identification results will be different at different times for the same substance, resulting in a degraded image quality.